Technical:

A category (in a wiki often called namespace) is under normal circumstances known as the first part of the page name - the partial name before the colon (:) in the page name. You will see this page name in the adress bar as the last part of the URL. This is NOT PART of the page title!
A page name without such colon in its name belongs in wikidot.com to the "_default" category.

As an example:http://community.wikidot.com/glossary:category is the URL of this article.

The category of this page is now "glossary",

the page name is "glossary:category",

and the page title is "category".

Be careful:http://community.wikidot.com/category would be a complete different URL of a page with (an unvisible) category name "_default"!!

Logical:

With such partial names you can "group" pages in your wiki into logical categories. Think for example of a group of uneditable – readonly – pages for all visitors and another group of pages where some types of visitors (anonymous, registered wikidot members or even site members) can edit or /view(!) the pages .

Typical (standard) categories in a wikidot.com site are "_default", "admin", "nav", "forum", "system", and "search". You should never delete them!

To create first time a new category:
A new category is built by creating (adding) a new page by entering thecategory name, a colon (:), and the rest of the page name either:

– as an open link in an article (usual in a wiki) (example of this page: [[[glossary:category]]] ) and after the save of the changed page you need to click on the "open link" to create the new page…

or

– key in this combined page name in the adress bar of your browser at the end of the url after the …wikidot,com/ - with enter to get the create a new page…

Setup and usage:
If you are an admin of a wiki you can see all used categories in the wiki on the Site manager (this is the admin:manage page in the category "admin") – under the "permissions", where you can decide for every category (and all its pages) if the visitors are allowed to edit such pages (= to see the page options buttons) or not.

And you can decide which group of visitors can "VIEW" the pages of such a "private" category.